To maintain elasticity and visual functions of an eyeball, it is necessary to maintain intraocular pressure (IOP) within a certain range. The level of intraocular pressure is related to production and drainage of aqueous humor inside the eyeball. The aqueous humor is produced in the posterior chamber by ciliary processes of the ciliary body, flows through the pupil into the anterior chamber, and then flows from the trabecular meshwork in the corner of the anterior chamber into Schlemm's canal, or flows through uveal tissue gaps and is then recycled into blood via veins. The aqueous humor provides oxygen and nutrients to anterior intraocular tissues and remove metabolic wastes therefrom. A balance between production and drainage of the aqueous humor determines the level of intraocular pressure. If too much aqueous humor is produced or the drainage path is blocked, the intraocular pressure rises. Excessively high intraocular pressure may compress and damage nerves to cause visual field defects and visual acuity reduction, thus resulting in so-called glaucoma.
According to estimates made by the World Health Organization (WHO) and the International Agency for the Prevention of Blindness (IAPB), by 2020, eighty million people worldwide will have glaucoma and more than ten million of them will be bilaterally blind because of the disease. People having excessively high intraocular pressure are in a high-risk group for glaucoma. Clinically, normal intraocular pressure ranges from about 10 to 21 mmHg. Measurement of intraocular pressure is an important factor in controlling development of glaucoma. However, diurnal and nocturnal fluctuation in the intraocular pressure vary from person to person. The intraocular pressure of a normal person fluctuates within 2 to 6 mmHg. A patient with glaucoma has larger intraocular pressure fluctuation, sometimes of more than 10 mmHg. The intraocular pressure measured at the patient's follow-up visit to the doctor is merely an intraocular pressure value at a specific time during a day and does not reflect the diurnal and nocturnal fluctuation in the intraocular pressure. The doctor cannot determine whether the patient's intraocular pressure is under control 24 hours a day based only on this value, and hence cannot determine, in real time, timing of administration, frequency of administration, prescriptions or dosages. Early symptoms of glaucoma are mainly caused by death of optic nerves due to excessively high intraocular pressure over a long period. Among current methods of treating glaucoma, the only one considered reliably effective and capable of effective monitoring is to lower the intraocular pressure. Therefore, an instrument capable of real-time and long-term detection or monitoring of intraocular pressure will contribute to clinical monitoring and treatment of early glaucoma.
However, in terms of current general clinical screening methods, follow-up visits are scheduled about every three months or every half year, and the intraocular pressure measured at each visit is nothing more than an intraocular pressure value at a specific time during that day and cannot truly reflect fluctuation in the intraocular pressure over a long period. Only by long-term and constant tracking of intraocular pressure values, the patient's complete intraocular pressure record can be available to the doctor, who is thus able to actually see fluctuation conditions of the patient's intraocular pressure so as to set target IOP/baseline and safety thresholds particularly for the patient. Therefore, a method is needed for allowing the patient to perform self-detection and self-management of intraocular pressure at home and for enabling long-term and constant tracking of the intraocular pressure.
In addition, among existing medical tonometers, air-puff type non-contact tonometry (NCT) are most extensively used. However, conventional NCT include laser optical alignment and force sensing systems that have complex architecture and large size and are also costly, which makes them hardly acceptable as tonometers for self-detection at home. In addition, applanation tonometers provide a portable and accurate intraocular pressure measurement means. However, during measurement, the applanation tonometers require direct contact with the patient's cornea, and local anesthesia on the patient's eye is necessary. Moreover, the applanation tonometers cannot be operated by the patient themselves. Hence, these devices are not very convenient in use. Recently, implantable intraocular pressure sensors have been launched one after another. While satisfying the need for continuous intraocular pressure monitoring, these devices must be implanted in the eye by surgery and are highly invasive, thus reducing patient acceptance.
Based on the above reasons, it is an important subject in ophthalmology to develop a tonometer that enables self-detection and self-management at home, that is easy and safe to use, that is highly precise, and that has warning and reminder functions in order to solve the aforementioned problems.